There is a need for inexpensive, quickly formed and accurate terrain elevation maps. It would be helpful if these terrain elevation maps could be constructed from radar sensor data taken from aircraft. The data could be obtained by flying over the area to be mapped while operating the radar sensor. The radar sensor data could then be employed together with the known or measured path of the aircraft to produce the needed terrain elevation map. Synthetic aperture radar would be useful in construction of such terrain elevation maps except for some limitations in the prior art.
In synthetic aperture radar the motion of the aircraft is employed to achieve greater angular resolution than obtainable by antennas that can be mounted on the aircraft. The motion of the aircraft permits formation of a synthetic antenna that is larger in size than the real antenna. This large synthetic antenna has an angular resolution corresponding to the angular resolution of an equally large physical antenna. The aircraft flies in a predetermined path and repeatedly transmits a radar signal. It is typical for this radar signal to be directed to the side of the flight path via a small antenna. This cross track view may be directly perpendicular to the flight path or at some angle less than perpendicular. The same antenna receives return echoes of the transmitted signal.
The return echo signals are processed in two dimensions. The time to receive an echo return from any particular piece of terrain corresponds to its slant range from the aircraft. This range is called slant range because it typically follows a downwardly slanting path from the aircraft to the ground. The echo returns also have differing Doppler frequencies. The motion of the aircraft imparts a Doppler frequency shift in the echo returns from the stationary ground. Areas directly ahead of the aircraft have a maximum closing Doppler frequency, those directly behind have a maximum opening Doppler frequency. Areas at varying angles between these extremes have intermediate Doppler frequencies. The combination of time of return and Doppler frequency permit production of a two dimensional feature map of resolution cells. Plural echo returns can be processed together with the known path and velocity of the aircraft to produce a terrain map.
There is a problem with this technique. This technique produces a position ambiguity. An echo signal with a particular time of return and Doppler frequency does not define a unique location. Regions of echo return times equal within the limits of measurement lie on a spherical shell centered at the antenna location. Regions of Doppler frequencies equal within the limits of measurement lie on a conical shell having its vertex at the antenna and its axis on the velocity vector of the aircraft. The intersection of these regions forms a vertically disposed circular annulus. Actual echo returns can only come from areas illuminated by the transmissions, so that the return areas are further limited to the solid angle cone of the antenna. Still this leaves an ambiguity in the location of the terrain forming the echo return. Terrain features having greater elevations are foreshortened because they have a reduced slant range compared with to regions of lower elevation.
Richman, U.S. Pat. No. 4,321,601, issued Mar. 23, 1982 and entitled "THREE DIMENSIONAL AZIMUTH-CORRECTING MAPPING RADAR" proposes a solution to this terrain elevation ambiguity problem. Richman employs two synthetic aperture radar antennas disposed a known distance apart on the aircraft. The antennas have the same look angles to cover the same terrain. Each antenna has its data processed in both slant range and Doppler frequency to identify and correlate echo returns from the same portions of terrain in the two antennas. The phase difference between the echo returns of the two antennas for the same resolution cell corresponds to the slant angle to the location producing that echo. Simple trigonometry permits computation of terrain elevation for a particular echo return from the slant angle, the known altitude of the aircraft and the measured slant range.
The technique of Richman measures the difference in slant range between the respective antennas and the target location by measuring the phase difference of the echo returns. The phase difference measurement introduces a 2.pi. ambiguity. That is, the actual slant range difference could include one or more factors of 2.pi. while yielding the same phase difference. This circular phase ambiguity produces an ambiguity in the terrain elevation calculated by this technique. A known technique called phase unwrapping can be used to reduce this ambiguity. The phase unwrapping technique requires a good signal to noise ratio and well behaved terrain yet still produces a bias ambiguity over the entire terrain mapped. There is a need in the art to provide a manner of reducing or eliminating the measurement ambiguity introduced in this interferometric determination of the terrain elevation.